The present embodiments relate to a method and a control sequence device for determining and using a magnetic resonance system control sequence.
In a magnetic resonance system, a body to be examined may be subjected with the aid of a basic field magnet system to a relatively high basic magnetic field of 3 or 7 Tesla, respectively, for example. A magnetic field gradient is applied with the aid of a gradient system. Radio-frequency excitation signals (RF signals) are sent out by suitable antenna devices over a high-frequency transmission system that is designed to lead to the nuclear magnetic resonance of specific atoms excited by the radio-frequency field being flipped by a defined flip angle in relation to magnetic field lines of the basic magnetic field. This radio-frequency excitation or the resulting flip angle distribution is also referred to below as core magnetization or magnetization. In the relaxation of the nuclear spin radio-frequency signals, magnetic resonance signals are emitted. The magnetic resonance signals are received by suitable receive antennas and further processed. Desired image data may be reconstructed from raw data acquired in this way. The radio-frequency signals for nuclear spin magnetization may be sent out by a bodycoil.
The bodycoil may include a birdcage antenna that includes a plurality of transmit rods arranged in parallel to the longitudinal axis around a patient space of a tomograph, in which a patient is located during examination. On an end-face side, the antenna rods are connected capacitively to each other in a ring shape.
Previously, bodycoils may have been operated in a homogeneous mode (e.g., a CP mode). A single temporal RF signal is issued to all components of the transmit antenna (e.g., all transmit rods of a birdcage antenna). The pulses may be transferred offset in phase to the individual components with a displacement adapted to the geometry of the transmit coil. For a birdcage antenna with 16 rods, for example, the rods may be activated with the same RF signal offset with a phase displacement of 22.5°. Such a homogeneous excitation results in a global radio-frequency exposure of the patient, which in accordance with the regulations, must be restricted, since too great a radio-frequency exposure may lead to damage to the patient. The radio-frequency exposure of the patient may be calculated in advance during the planning of the radio-frequency pulses to be output, and the radio-frequency pulses are selected so that a specific threshold is not reached. A measure of radio-frequency exposure may be the specific absorption rate (SAR) value, which specifies in Watts/kg the biological stress, to which the patient is exposed by a specific radio-frequency pulse output. For a global SAR of a patient, a standardized limit of 4 Watts/kg at the First Level applies in accordance with the IEC standard. As well as the advance planning, the SAR exposure of the patient is monitored continuously during the examination by suitable safety devices on the magnetic resonance system, and a measurement is changed or aborted if the SAR value lies above intended standard levels. Accurate planning is sensible in order to avoid such an interruption of a measurement, since the interruption may make a new measurement necessary.
With more recent magnetic resonance systems, the individual transmit channels (e.g., the individual rods of the birdcage antenna) may be provided with RF signals adapted individually to the imaging. A multichannel pulse train that includes a plurality of individual radio-frequency pulse trains may be sent out in parallel over the various independent radio-frequency transmit channels. The multichannel pulse train, because of the parallel transmission of the individual pulses, may be referred to as a pTX pulse and may be used as an excitation, refocusing and/or inversion pulse.
Such multichannel pulse trains may be generated in advance for a specific planned measurement. The individual pulse trains (e.g., the RF trajectories) are determined in an optimization method for the individual transmit channels over time as a function of a fixed k-space gradient trajectory that may be predetermined by a measurement protocol. The transmit k-space gradient trajectory (abbreviated below simply to k-space gradient trajectory or gradient trajectory) includes the locations in the k-space that are reached at specific times by setting the individual gradients. The k-space is the local frequency space, and the gradient trajectory in the k-space describes the path on which the k-space is traveled during the transmission of an RF pulse or the parallel pulse through corresponding switching of the gradient pulses over time. By setting the gradient trajectory in the k-space (e.g., by setting the suitable gradient trajectory applied in parallel to the multichannel pulse train), the local frequencies and which specific RF energy amounts will be deposited may be defined.
During the construction of the gradient trajectory, relevant areas in the k-space are also traversed. For example, if a sharply defined region (e.g., a rectangle or oval) is to be excited in the local space, the k-space may also be well covered in an outer boundary area. However, for an unsharp delimitation, coverage in the central k-space area is sufficient.
The user also prespecifies a target magnetization of the RF pulse train for the planning (e.g., a desired flip angle distribution).
With a suitable optimization program, the appropriate RF pulse train is calculated for the individual channels so that the target magnetization is achieved. The protocol developer may also already have a certain level of experience in selecting the k-space trajectory, so that the target magnetization may be achieved thereby. A method for developing such multichannel pulse trains in parallel excitation methods is described, for example, in W. Grishom et al.: “Spatial Domain Method for the Design of RF Pulses in Multicoil Parallel Excitation,” Mag. Res. Med. 56, 620-629, 2006.
For a specific measurement, the various multichannel pulse trains, the gradient pulse train to be transmitted coordinated with the various multichannel pulse trains (with suitable x, y and z gradient pulses) and further control requirements are defined in a measurement protocol. The measurement protocol is created in advance and may be retrieved for a specific measurement from a memory and may be modified by the operator locally. During the measurement, the magnetic resonance system is controlled fully automatically on the basis of the measurement protocol, with the control device of the magnetic resonance system reading out the commands from the measurement protocol and processing the commands in turn.
During the transmission of multichannel pulse trains, in the measurement space and thus in the patient, the previously homogenous excitation may be replaced by an excitation formed in any given way. To estimate the maximum radio-frequency exposure, each possible radio-frequency overlay is investigated. Each possible radio-frequency overlay may, for example, be investigated using a patient model including properties typical of tissues (e.g., conductivity, dielectricity, and density) in a simulation. It may be known from previous simulations that hotspots may form in the radio-frequency field in the patient. The radio-frequency exposure may amount to a multiple of the values previously known from the homogenous excitation at the hotspots. The resulting radio-frequency limitations are otherwise unacceptable for the performance of clinical imaging, since the overall transmit power would be too low to create acceptable images if account were taken of the hotspots. The radio-frequency exposure may be reduced during transmission of the multichannel pulse trains.